The construction and operation of conical loudspeakers are well known in the state of the art, so that a broader discussion can be omitted in this instance.
The magnet system of such conical loudspeakers usually consists of a ring-shaped permanent magnet, an upper and a lower pole plate and a pole core. The pole core is centrally located on the lower pole plate, and is surrounded at a distance by the ring-shaped permanent magnet, the ring surface of which is also connected to the lower pole plate. The upper, equally ring-shaped pole plate is located on the other ring surface of the permanent magnet. The length of the pole core is so dimensioned, that the free end of the pole core, which is not connected to the lower pole plate, closes off the upper pole plate when the magnet system is installed, where the inside border of the upper pole plate surrounds the pole core at a distance. The voice coil, which is connected to the loudspeaker diaphragm, enters into this gap, commonly called an air gap. Except for permanent magnets made of ferrite, which already receive the necessary shape during the sintering process, all other components of the magnet system are either stamped or extruded.
A disadvantage of such magnet systems is that such loudspeakers are very heavy. This can be attributed to the fact that large, and therefore heavy, permanent magnets are required to produce sufficient induction in the air gap. This applies particularly when loudspeakers with large excursion voice coils are driven by such magnet system. In that instance it is necessary to select a larger winding width of the voice coil and a greater height of the pole plate, than in situations where the magnet system is designed for short excursion reproduction. The result is that, because of the greater thickness of the upper pole plate or the larger winding width of the voice coil in such long excursion magnet systems, the required induction in the air gap can only be produced by the superproportional enlargement of the permanent magnet, as compared to short excursion systems.
In addition to this magnet system configuration, high and mid-range magnet systems are known, which differ in construction from the above-named magnet systems. Such magnet systems have a pot magnet, on the bottom of which the pole core is centrally located with respect to the loudspeakers axis. The pole core in this configuration is made of a high energy magnetic material, known as neodymium. The end of the pole core that is not connected to the bottom of the pot is equipped with an upper pole plate, which has a larger diameter than the pole core. The height of the pot edge coincides with the height of the pole core and the pole plate. The pot edge and the components in the pot (pole core and pole plate) are usually of the same height. The magnet system's air gap is formed between the upper pot edge and the pole plate, since the two components are at a distance from each other. The upper part of the pot edge, in other words the part facing the pole plate, can be pole-shaped, by letting this part of the pot edge protrude inside the pot. Such magnet systems have the advantage that they can be built clearly lighter, as opposed to comparable loudspeakers with permanent magnets made exclusively of ferrite. However, this applies only to magnet systems that operate with short excursion. These are above all high and mid-range loudspeakers. This can be attributed to the fact that, in spite of the superior characteristics of neodymium, the required induction in the air gap is only suitable for narrow winding widths of the voice coil. Furthermore, the neodymium magnet system has the disadvantage that the pot magnet must be turned on a lathe, and is therefore more expensive to produce than the above described pure ferrite systems. However, even if this disadvantage is not taken into consideration, and it is attempted to also build long excursion magnet systems in the above described manner, it is not possible to transfer the neodymium magnet systems used in short excursion operation to the long excursion magnet systems. The reason is that this type of loudspeaker requires a larger constructed size and greater induction in the air gap, because of the larger excursion, as compared to high and mid-range loudspeakers. This cannot be achieved satisfactorily by enlarging the neodymium pole core.
For that reason, the invention has the task of providing a conical loudspeaker, in particular a magnet system for long excursion conical loudspeakers, which has a clearly lower weight by comparison to known magnet systems whose permanent magnets are made exclusively of ferrite.